How to choose between entering a Radius or a Diameter when creating a Circle
It's not posible to toggle the Radius field to Diameter. However, as the diameter is twice the radius there is a direct relationship between the two - so the Radius is the Diameter divided by 2. Also, Blender is capable of automatically interpreting mathematical formula when entering values.

Therefore, to create a circle of, say, 30 units in diameter, simply type 30/2 into the Radius field and Blender will automatically calculate 15 as the radius.